1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to an injection molding machine that detects, displays, and predicts a wear amount or a closing stroke of a check valve.
2. Description of the Related Art
An in-line screw type injection molding machine has a check valve unit on a screw tip to prevent backflow of resin while the screw advances.
Resin that is melted during a metering stage is accumulated in front of the screw inside a heated barrel. Injection and pressure holding stages are begun, the screw advances, and the accumulated resin is injected into a mold and fills the interior of the mold. During a time period extending from the start of injection to a time when a check valve closes a resin flow channel, the resin flows back from the front of the check valve toward the rear. The amount of this backflow affects the amount of resin filling the mold, and therefore affects the quality of the molded article.
As the injection molding machine continues to be used over a long period of time, the check valve begins to wear due to corrosion caused by the resin and by friction during rotation of the screw. As a result, the backflow amount changes, affecting the quality of the molded article.
Consequently, a variety of methods for detecting check valve wear and resin backflow amounts are known.
For example, methods are known that detect the backflow amount based on the distance over which the screw advances during pressure holding (JP62-3916A) or on the speed with which the screw advances during pressure holding (JP01-281912A) and generate a warning when the detected backflow amount exceeds a predetermined value so as to prevent defective molding, detect wear on the check valve, and the like.
In addition, a method is known that takes advantage of the fact that, once resin backflow occurs when the screw is advanced, a force equivalent to a backflow resin pressure (that is, an injection pressure force) pushes against flights of the screw. This force is divided into a force exerted in the axial direction of the screw (a force that pushes backwards) and a force exerted in the direction of rotation of the screw (a force that causes the screw to rotate in the reverse direction of the direction in which the screw rotates during metering). Therefore, the resin backflow causes a screw rotational force equivalent to the injection pressure force to be exerted on the screw, and once this screw rotational force exceeds a predetermined value it is determined that there is something wrong with the check valve, in the form of wear, damage, or the like (see, for example, JP01-168421A).
In the background art described above, the methods for detecting check valve wear involve detecting the resin backflow amount or the like and outputting an alarm when that backflow amount equals or exceeds a predetermined value. However, check valve wear proceeds gradually, and therefore, at the stage when an alarm is given, the injection molding machine has already formed large quantities of defective molded articles, which is undesirable. Accordingly, it is desirable to be able to grasp the extent of wear on the check valve, and take the necessary steps to cope with the situation by ascertaining that the check valve has reached its use limit, before the alarm is given.